1. Field of the Invention
The invention relates to a method for classifying polysilicon.
2. Description of the Related Art
Polycrystalline silicon (polysilicon for short) serves as a starting material for production of monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone-melting (FZ) methods, and for production of mono- or multicrystalline silicon by various pulling and casting methods for production of solar cells for photovoltaics.
Polycrystalline silicon is generally produced by means of the Siemens process. This process involves heating support bodies, typically thin filament rods of silicon, by direct passage of current in a bell jar-shaped reactor (“Siemens reactor”), and introducing a reaction gas comprising hydrogen and one or more silicon-containing components. Typically, the silicon-containing component used is trichlorosilane (SiHCl3, TCS) or a mixture of trichlorosilane with dichlorosilane (SiH2Cl2, DCS) and/or with tetrachlorosilane (SiCl4, STC). Less commonly, but also on the industrial scale, silane (SiH4) is used. The filament rods are inserted vertically into electrodes present at the reactor base, through which they are connected to the power supply. High-purity polysilicon is deposited on the heated filament rods and the horizontal bridge, as a result of which the diameter thereof increases with time. After the rods have been cooled, the reactor bell jar is opened and the rods are removed by hand or with the aid of specific devices, called deinstallation aids, for further processing or for intermediate storage. For most applications, polycrystalline silicon rods are broken into small chunks, which are usually then classified by size.
Polycrystalline silicon granules or granular polysilicon for short is an alternative to the polysilicon produced in the Siemens process. While the polysilicon in the Siemens process is obtained as a cylindrical silicon rod which has to be comminuted to chunks in a time-consuming and costly manner and may need to be cleaned before further processing thereof, granular polysilicon has bulk material properties and can be used directly as raw material, for example for single crystal production for the photovoltaics and electronics industries. Granular polysilicon is produced in a fluidized bed reactor. This is accomplished by fluidization of silicon particles by means of a gas flow in a fluidized bed, the latter being heated to high temperatures by means of a heating device. Addition of a silicon-containing reaction gas results in a pyrolysis reaction at the hot particle surface. This causes deposition of elemental silicon on the silicon particles and growth in the individual particle diameter. Through the regular removal of particles that have increased in size and addition of small silicon particles as seed particles, it is possible to operate the process continuously with all the associated advantages. Silicon-containing reactant gases used may be silicon-halogen compounds (e.g. chlorosilanes or bromosilanes), monosilane (SiH4), and mixtures of these gases with hydrogen.
After they have been produced, the polycrystalline silicon granules are divided into two or more fractions by means of a screening system.
The smallest screen fractions (screen undersize) can subsequently be processed in a grinding system to give seed particles and added to the reactor.
The target screen fraction is typically packed.
US 2009081108 A1 discloses a workbench for manual sorting of polycrystalline silicon by size and quality. This implements an ionization system to neutralize electrostatic charges by active air ionization. Ionizers permeate the cleanroom air with ions such that static charges at insulators and ungrounded conductors are dissipated.
Typically, screening machines are used to sort or to classify polycrystalline silicon into different size classes after comminution. A screening machine is generally a machine for screening, i.e. separation of solid mixtures by particle size. A distinction is made by the movement characteristics between planar vibratory screening machines and gravity screening machines. The screening machines are usually driven electromagnetically or by imbalance motors or drives. The movement of the screen lining serves to transport the material applied onward in the longitudinal direction of the screen, and for passage of the fines fraction through the mesh orifices.
In contrast to planar vibratory screening machines, a vertical screen acceleration also occurs as well as the horizontal screen acceleration in gravity screening machines. In the gravity screening machines, vertical throwing motions are combined with gentle rotary motions. The effect of this is that the sample material is distributed over the whole area of the screen deck and the particles simultaneously experience acceleration in the vertical direction (are thrown upward). In the air, they can perform free rotations and, when they fall back down onto the screen, are compared with the meshes of the screen fabric. If the particles are smaller than these, they pass through the screen; if they are larger, they are thrown upward again. The rotating motion ensures that they will have a different orientation the next time they hit the screen fabric, and thus will perhaps pass through a mesh orifice after all.
In planar screening machines, the screening tower performs a horizontally circular motion in a plane. As a result, the particles for the most part retain their orientation on the screen fabric. Planar screening machines are preferably used for acicular, platelet-shaped, elongated or fibrous screening materials where throwing of the sample material upward is not necessarily advantageous.
A specific type is the multideck screening machine, which can simultaneously fractionate several particle sizes. They are designed for a multitude of sharp separations in the mid-grain to ultrafine-grain range. The drive principle in multideck planar screening machines is based on two imbalance motors running in opposite directions, which generate a linear vibration. The screening material moves in a straight line over the horizontal separation surface. The machine works with low vibratory acceleration.
The drive principle in multideck planar screening machines is based on two imbalance motors running in opposite directions, which generate a linear vibration. The screening material moves in a straight line over the horizontal separation surface. The machine works with low vibratory acceleration.
Through a building block system, a multitude of screen decks can be assembled to form a screen stack. Thus, if required, different particle sizes can be produced in a single machine without needing to change screen linings. Through multiple repetition of identical screen deck sequences, it is possible to make a large amount of screen area available to the screening material.
U.S. Pat. No. 8,021,483 B2 discloses an apparatus for sorting polycrystalline silicon pieces, comprising a vibratory motor assembly and a step deck classifier mounted to the vibratory motor assembly. The vibratory motor assembly ensures that the silicon pieces move over a first deck comprising grooves. In a fluidized bed region, dust is removed by an air stream through a perforated plate. In a profiled region of the first deck, the silicon pieces settle into the troughs of the grooves or remain on top of the crests of the grooves. As the polycrystalline silicon pieces reach the end of the first deck, silicon pieces smaller than the gap fall through the gap and onto a conveyor belt. Larger silicon pieces pass over the gap and fall onto the second deck. The parts of the apparatus that come into contact with the polycrystalline silicon pieces consist of materials that minimize contamination of silicon. Examples mentioned include tungsten carbide, PE, PP, PFA, PU, PVDF, PTFE, silicon and ceramic.
US 2007235574 A1 discloses a device for comminuting and sorting polycrystalline silicon, comprising a means for feeding a coarse polysilicon fraction into a crushing system, the crushing system, and a sorting system for classifying the crushed polysilicon fraction, wherein the device is provided with a controller which allows variable adjustment of at least one crushing parameter in the crushing system and/or at least one sorting parameter in the sorting system. The sorting system more preferably consists of a multistage mechanical screening system and a multistage optoelectronic separating system. Vibrating screen machines are preferably used, which are driven by an unbalance motor. Meshed and perforated screens are preferred as a screen lining.
The screening stages may be arranged in series or in another structure, for example a tree structure. The screens are preferably arranged in three stages in a tree structure. The crushed polysilicon fraction freed from fine components is preferably sorted by means of an optoelectronic separating system. The polysilicon fraction may be sorted according to all criteria which are known in image processing in the prior art. It is preferably carried out according to one to three criteria selected from the group of length, area, shape, morphology, color and weight of the polysilicon fragments, more preferably length and area.
This enables the production of the following fractions:
Fraction 0: chunk sizes with a distribution of approximately 0 to 3 mm
Fraction 1: chunk sizes with a distribution of approximately 1 mm to 10 mm
Fraction 2: chunk sizes with a distribution of approximately 10 mm to 40 mm
Fraction 3: chunk sizes with a distribution of approximately 25 mm to 65 mm
Fraction 4: chunk sizes with a distribution of approximately 50 mm to 110 mm
Fraction 5: chunk sizes with a distribution of approximately >90 mm to 250 mm
There is no information as to the exact distribution of the chunk sizes within the fractions in US 2007235574 A1.
U.S. Pat. No. 5,165,548 A discloses a device for separating semiconductor grade silicon pieces by size, comprising a cylindrical screen contacted with a means for rotating the cylindrical screen, where the screen surfaces that come into contact with the silicon pieces consist essentially of semiconductor grade silicon.
U.S. Pat. No. 7,959,008 B2 claims a method for screening first particles out of a granulate comprising first and second particles by conveying the granulate along a first screen surface preferably emanating from a vibration unit, wherein the first particles have an aspect ratio a1 where a1>n:1 and n=2, 3, >3, especially with a1>3:1, and the dimensions of the second particles allow them to fall through the mesh of the first screen surface, wherein the granulate is conveyed along the screen surface between said surface and a cover which extends along the screen surface, and the cover causes the first particles to be aligned with their longitudinal axes extending along the screen surface, wherein the longitudinal extension of each first particle is greater than the mesh width of the screen which forms the first screen surface, and the longitudinal extension of the second particles is equal to or smaller than the mesh width.
EP 1454679 B1 describes a screening apparatus having a first vibrating body provided with first crossmembers, and a second vibrating body provided with second crossmembers, which first and second crossmembers are positioned in alternation and have clamping devices so that elastic screen linings may be clamped between one first crossmember and one second crossmember in each case, and have a drive unit which is directly coupled to the first vibrating body and by means of which the first vibrating body is positively driven, so that the clamped elastic screen linings are moved back and forth between a stretched position and a contracted position, the second vibrating body being positively driven with respect to the first vibrating body.
U.S. Pat. No. 6,375,011 B1 discloses a method for conveying silicon fragments wherein the silicon fragments are guided over a conveyor surface, which is made from hyperpure silicon, of a vibrating conveyor. In the course of this method, sharp edged silicon fragments become rounded when they are conveyed on the vibrating conveyor surface of a vibrating conveyor. The specific surface areas of the silicon fragments are reduced; contamination adhering to the surface is ground off. The silicon fragments which have been rounded by means of a first vibrating conveyor unit can be guided over a second vibrating conveyor unit. The conveyor surface thereof consists of hyperpure silicon plates which are arranged parallel to one another and are fixed by means of side attachment fittings. The hyperpure silicon plates have passage openings, for example in the form of apertures. The conveying edges, which serve to laterally delimit the conveyor surfaces, are likewise made from hyperpure silicon plates and are fixed, for example, by holding-down means. The conveyor surfaces, which are made from hyperpure silicon plates, are supported by steel plates and, if appropriate, shock-absorbing mats.
US 2012052297 A1 discloses a method for producing polycrystalline silicon, comprising fracturing into fragments polycrystalline silicon deposited on thin rods in a Siemens reactor, classifying the fragments into size classes of from about 0.5 mm to more than 45 mm, treating the silicon fragments with compressed air or dry ice to remove silicon dust from the fragments without wet chemical cleaning. The polycrystalline silicon is classified as follows: chunk size 0 (CS0) in mm: about 0.5 to 5; chunk size 1 (CS1) in mm: about 3 to 15; chunk size 2 (CS2) in mm: about 10 to 40; chunk size 3 (CS3) in mm: about 20 to 60; chunk size 4 (CS4) in mm: about >45; with at least 90% by weight of the chunk fraction within each size range mentioned. This corresponds to the specification of the different chunk sizes into which the silicon is to be classified. The application does not give any information as to the actual result of the classification or sorting of the silicon and the size distributions within the individual size classes.
US 2009120848 A1 describes a device which enables flexible classification of crushed polycrystalline silicon, which comprises a mechanical screening system and an optoelectronic sorting system, the polycrystalline silicon fragments being separated into a fine silicon component and a residual silicon component by the mechanical screening system and the residual silicon component being separated into further fractions by means of an optoelectronic sorting system. The mechanical screening system is preferably a vibratory screening machine which is driven by an imbalance motor.
In the course of mechanical classification by screening by means of vibratory screening machines according to the prior art, material worn away from the screen lining is introduced into the product. This results in contamination of the polysilicon with constituents present in the screen lining. Another disadvantage in the prior art is that the fractions into which the polysilicon is classified have a distinct overlap. In the prior art, a certain overlap in the specifications has already been accepted.
In US 2012052297 A1, the overlap between chunk size 2 and chunk size 1 is max. 5 mm, and that between chunk size 1 and chunk size 0 is max. 2 mm. This relates to the specification to which classification is to be effected. The actual distribution of the chunk sizes is generally different from this.
According to US 2007235574 A1, the overlap between a fraction 1 and a fraction 0 is likewise max. 2 mm. Particularly in the case of fractions with smaller chunk sizes of 30 mm or less, such an overlap is undesirable.
This problem gave rise to the objective of the invention.